Object Transform Origin to Geometry Floor
Enable the align tools Add-On.

In the 3D Viewport select your objects, open the Sidebar (N)
Go to the Item tab.
In  Align Tools &gt; Advanced Align Operations, click on the Advanced button.
On the bottom left of the viewport window the Align operators control will open.
Set the Operator to Pivot mode.
Enable Active too, Self and Min
Then  set the Axis you are aligning, in this case it should be $Z$

Then your objects origins are at the bottom.
For older versions of Blender the interface looks like this: